The present invention relates to a novel process for improving the qualities of paper. More particularly, it relates to a process for improving the qualities of paper in which the paper strength, the retention of sizing agents and the retention of fillers are markedly improved by employing a water-soluble polymer containing hydrazide group or its adduct with a water-soluble inorganic salt of an alkaline earth metal in combination with a specific heavy metal ion in a pulp slurry.
It is known that a water-soluble polymer containing acrylic or methacrylic hydrazide units in its polymer chain behaves as a cationic polymer in an acidic aqueous solution due to the presence of hydrazide group and is effective as a retention aid for sizing agents or a strengthening agent for paper. For instance, it is reported that the strengthening effect of the polymer is several times as good as that of .alpha.-starch and the sizing degree obtained by employing the polymer is several tens times as good as that obtained by employing a melamine-formaldehyde condensation product (see "J. Japanese TAPPI", Vol. 19, pages 440 to 445 (1965), Japan).
Such as acrylic or methacrylic hydrazide polymer (hereinafter referred to as "(meth)acrylic hydrazide polymer") is not absolutely protonated at a pH value of not less than 3.5. Therefore, when the polymer is employed in paper making, it should be protonated by adjusting the pH value of a pulp slurry to less than 3.5 with an acid such as hydrochloric acid. However, such a protonation employing hydrochloric acid causes a fatal problem that paper machines or dryers are corroded and further problems such as the indispensability of the neutralization of waste water because of its low pH value. For this reason, the (meth)acrylic hydrazide polymers have not been effectively utilized, despite that their usefulness is known.